The present invention relates to treatments for vascular diseases and other diseases of body lumens, in particular to a method of manufacturing a stent.
The inventions described below were developed with the goal of providing new and better therapies for certain types of vascular disease for which the present day therapies are widely regarded as inadequate. Vascular disease includes aneurysms which can rupture and cause hemorrhage, atherosclerosis which can cause the occlusion of the blood vessels, vascular malformation and tumors. Occlusion of the coronary arteries, for example, is a common cause of heart attack. Vessel occlusion or rupture of an aneurysm within the brain are causes of stroke. Tumors fed by intra-cranial arteries can grow within the brain to the point where they cause a mass effect. The mass and size of the tumor can cause a stroke or the symptoms of stroke, requiring surgery for removal of the tumor or other remedial intervention.
The newly preferred therapy for coronary occlusions is placement of an expanded metal wire-frame, called a stent, within the occluded region of the blood vessel to hold it open. Stents of various construction have been proposed, including the Palmaz-Schatz(trademark) balloon expandable metal stent, the Wallstent self-expanding braided metal stent, the Strecker knitted metal stent, the Instent(trademark) coil stent, the Cragg coiled stent and the Gianturco Z stent. Stents have been proposed for treatment of atherosclerosis in the neck, but carotid endarterectomy is still the preferred treatment for stenosis. Most perioperative strokes are thought to be caused by technical errors during endarterectomy (see Becker, Should Metallic Vascular Stents Be Used To Treat Cerebrovascular Occlusive Disease, 191 Radiology 309. (1994)). The same concerns militate against other forms of therapy such as angioplasty for treatment of the carotid arteries. Various factors, including poor long-term patency, distal emboli causing a stroke, the potential for crushing from external pressure, and the need for long term anti-coagulation, lead to the avoidance of certain stents in vessels smaller than the iliac arteries or in locations susceptible to external pressure. See, for example, Hull, The Wallstent in Peripheral Vascular Disease, For Iliac Use Only, 6 JVIR 884 (November-December 1995).
Stent-grafts have been proposed and used to treat aneurysms in the large blood vessels such as the aorta, and these typically include tube graft material supported by a metallic stent. These stent-grafts are designed for use in the large blood vessels, and the various layers of stents and grafts make them unsuitable for use in smaller blood vessels. Stent-grafts are not currently used in the coronary arteries which are typically 3 or 4 mm in internal diameter. Rolled stents have been proposed for use in aortic aneurysms. For example, Lane, Self Expanding Vascular Endoprosthesis for Aneurysms, U.S. Pat. No. 5,405,379 (Apr. 11, 1995) suggests the use of a polypropylene sheet placed in the abdominal or thoracic aorta to bridge aneurysms. Winston, Stent Construction of Rolled Configuration, U.S. Pat. No. 5,306,294 (Apr. 26, 1994) proposes a rolled sheet of stainless steel. Of similar construction are the single layer rolled stents such as Kreamer, Intraluminal Graft, U.S. Pat. No. 4,740,207 (Apr. 26, 1988) and its reissue Pat. No. Re 34,327 (Jul. 27, 1993), which are expanded by balloon and include a ratchet mechanism which projects into the lumen of the stent. Khosravi, Ratcheting Stent, U.S. Pat. No. 5,441,155 (Aug. 15, 1995) and Sigwart, Intravascular Stent, U.S. Pat. No. 5,443,500 (Aug. 22, 1995) are other examples of rolled stents with ratcheting locking mechanisms.
Stents have not previously been used for aneurysms of the blood vessels in the brain. The vessels in the brain likely to develop stenosis, aneurysms, AVM""s and side branches requiring occlusion have diameters of about 1 mm to 5 mm, and can be accessed only via highly tortuous routes through the vascular system. The stents described below will be delivered percutaneously, introduced into the body through the femoral artery, steered upwardly through the aorta, vena cava, carotid or vertebral artery, and into the various blood vessels of the brain. Further insertion into the brain requires passage through the highly tortuous and small diameter intra-cranial blood vessels. The Circle of Willis, a network of blood vessels which is central to the intracranial vascular system, is characterized by numerous small arteries and bends. Passage of a stent from the internal carotid through the Circle of Willis and into the anterior cerebral artery (for example) requires a turn of about 60xc2x0 through blood vessels of only 1-5 mm in diameter. Clinically, many significant aneurysms take place in the Circle of Willis and approaching blood vessels. The stent produced according to the methods described herein are intended for use in such highly tortuous vessels, particularly in the Circle of Willis, the vertebral and carotid siphons and other major blood vessels of the brain. At times, pathologically tortuous vessels may be encountered in the deeper vessels of the brain, and these vessels may be characterized by small diameter, by branching at angles in excess of 90xc2x0 and by inaccessibility with guide wires larger than the standard 0.018 guide-wires. These pathologically tortuous vessels may also be subject to aneurysms and AVM""s which can be treated with the stents produced according to the methods described below.
In order to fabricate sheet stents of extreme thinness, we have cold rolled metals such as Elgiloy, nitinol and stainless steel. Rolling appears to be effective to provide sheets of thickness down to 0.0011 inches. In order to fabricate thinner sheets, we have used chemical etching techniques to etch away even more of the sheet. This technique has enabled construction of sheets as thin as 0.0005xe2x80x3 with somewhat uniform thickness. The method of constructing the stent described below, and the stent resulting from this method, will provide rolled sheet stents made according our prior disclosures in smaller and thinner dimensions than was previously possible. The fabrication method may be applied to all the stents previously used and proposed in the art, with the added advantage that stent is provided in a much thinner and/or stronger form, and may be constructed of nitinol or other metals and materials in a manner not previously used.
In the far-afield arts of micro-machines and microactuators, the fabrication of shape memory switches of microscopic proportions has been proposed. The fabrication technique is called sputter deposition. The resultant material is referred to as a thin film. The sputter-deposited films have been experimentally used in micro-valves and micro-grippers. Thin film sputtering processes are used in the manufacture of microchips to lay down very small and very thin circuit lines on circuit substrates such as silicon chips. Thin film processes are used to coat plastic articles with decorative chrome finishes. In general, thin film sputter techniques use high power electromagnetic fields to create energetic particles or photons (plasma ions, ion beams, electron beam, laser beam) directed toward a target plate of the coating material to dislodge single atoms or molecules of the coating material onto a substrate. The dislodged atoms or molecules condense on the substrate and adhere very strongly to the substrate. The sputter process is usually performed at very high temperature of one hundred to several hundred degrees centigrade, and performed within an atmosphere of very high vacuum and/or an atmosphere of an inert gas. Sputter techniques are part of a broader field of processes referred to as physical vapor deposition or PVD techniques. The PVD processes are part of the broader field of thin film deposition, which also includes chemical vapor deposition, and electroplating. The key to all these processes is the placement of the substrate (the article to be coated) in an atmosphere or cloud of film molecules.
The rolled sheet stents which we have proposed for use in very small blood vessels of the brain may be constructed according to thin film sputtering techniques. Rather than roll metals to the desired thickness, which may be on the order of several thousands of an inch and thinner, the stent is created by sputtering hot molten metal onto a mold substrate. This results in a stronger sheet of metal vis-à-vis the rolling process. Rather than mechanically or photochemically cut the desired perforations into the sheet, the perforations are formed during the sputtering process as areas which are not sputtered. When the stent is made of nitinol by thin film sputtering techniques, the resultant sheet may be cured at high temperature to provide a stent with the same pseudoelastic or shape memory properties as found in bulk prepared rolled sheets of nitinol. Thin sheets of nitinol, with a uniform thickness of 0.0002 inches and less, can be made with this technique. The specific embodiment of thin film deposition used to exemplify the invention is RF powered physical vapor deposition. However, the various techniques of thin film deposition may be used.
Rolled sheet stents are preferably provided with dense perforation patterns, as illustrated in Wallace, et al, Intracranial Stent and Method of Use, PCT App. PCT/US97/16534. These perforation patterns have been made using a photochemical machining process which includes coating the sheet with a photoresist coating, processing the photoresist coating to remove the coating in areas corresponding to the desired perforations, thereby creating a partial coating which is a reverse image of the desired perforation pattern on the sheet, etching the metal in the uncoated areas with a chemical etchant to remove the metal in the areas corresponding to the desired perforation pattern, and then stripping the photoresist coating. This allows the creation of such thin sheets of metal without resort to mechanical cutting. By manufacturing the stent with a sputtering technique, the perforation patterns can be created by control of the deposition of the sputtered metal, thus eliminating the need for that the stent be subjected to photochemical machining process after its formation.